Robots are about to get more feeling. An electronic skin as sensitive to touch as our own is being developed by scientists in Japan.

"Recognition of tactile information will be very important for future generations of robots," says Takao Someya at the University of Tokyo who developed the skin. A sense of touch would help them to identify objects, carry out delicate tasks and avoid collisions. But while a lot of effort has gone into vision and voice recognition for robots, touch sensitivity is still fairly rudimentary.

Our own skin contains a battery of touch receptors that produce nerve signals when pressed. For gentle pressures, the main sensors are tiny bulbs of layered tissue called Meissner's corpuscles.

Their behaviour is mimicked in plastics such as polyvinylidene fluoride, which generate an electric field when squeezed and are used to make pressure-sensitive pads for computer keyboards and other touch-triggered devices.

Sensory perception

But for an electronic skin to have a genuine sense of touch, it needs to be able not just to sense pressure, but to know where it is being applied. So the skin must be covered with a whole bank of individual sensors, each of which sends a signal when pressed.

Someya and colleagues have wired up such a skin11. It consists of a sheet of rubbery polymer, impregnated with flakes of electrically conducting graphite. The electrical resistance of the sheet changes when it is squeezed, and this change is detected by an array of transistors beneath the rubber.

The key challenge is to make the whole device sufficiently flexible to behave like a true skin, so that it can be wrapped around a robotic limb. Conventional transistors in microchips are hard, brittle devices made from silicon. But Someya and colleagues have made them from an flexible organic material called pentacene. Their sensor array consists of a 32 x 32 grid of transistors, each of which is 2.5 mm square. The researchers are hopeful that they will be able to make transistors 100 times smaller than that if necessary.

The resulting skin can be bent quite sharply without damaging the transistors, and continues to function even when wrapped around a bar just 2 mm wide.

Stretchy skin

Of course our own sense of touch doesn't rely on pressure alone; we can also detect temperature and humidity, for example. The Japanese team hopes to add such features to their artificial skin. They also want to make it stretchy. At present it is more like a sheet of paper; bendy but not elastic.

"The absence of good devices of this sort has made it very difficult to move forward", comments Robert Howe, who works on tactile sensing for robotics at Harvard University in Cambridge, Massachusetts. But he reserves judgement about the impact of the new electronic skin. "Many such devices have been proposed, but none seems to make it beyond the lab bench."

Meanwhile, Someya is confident that his skin could find many applications beyond robotics, for example in sport, security or medicine. A pressure-sensitive carpet on the floor of a house could distinguish family members from strangers just from their footprints, he suggests, or sense if an elderly person had collapsed on it. Tactile mats could monitor the performance of athletes in the gym, while tactile seat coverings might measure the physical condition of car drivers.

References

Someya, T. et al. Proceedings of the National Academy of Sciences of the USA, Advance Online Publication,(2004) http://www.pnas.org/cgi/content/abstract/0401918101